Needle shields for syringes are well known and typically include a rigid plastic shell or cover with a soft plug or sheath therein. The plug and rigid plastic shell are either separately constructed and assembled by positioning the plug in the shell to form a needle shield or the plastic shell is constructed and the plug is injection molded into the shell to form the needle shield. The needle shield is removably mounted to the needle end of a syringe to protect a user from pricks or sticks from the needle and to avoid exposure of the needle to contaminants. U.S. Pat. No. 4,986,818 discloses a typical needle shield that is removably mountable to a syringe to cover the needle tip and generally protect the needle tip from contamination because the tip is located in the plug in a storage configuration. The needle tip and needle are also stabilized by the plug in the storage configuration to generally prevent damage to the needle and tip and to limit user exposure to the needle tip.
Conventional needle shields constructed utilizing an assembly of a preformed plug positioned in a rigid plastic shell often result in disassembly of the plug and shell when removing the syringe and syringe needle from the needle shield assembly. The plug and shell may become disconnected from one another during removal of the needle from the plug because disassembly forces of the needle shield from the syringe may be quite high over the shelf life of the assembly. In addition, the plug and/or shell may break during disassembly of the syringe and syringe needle from the assembly. For example, the rigid, plastic needle shell or plug may stick to a glass syringe or to the needle making it difficult for a user to remove the needle shield from the syringe, resulting in elevated removal forces. The stickiness or adhesion of the needle shield to the syringe or needle becomes more prominent over time. Accordingly, the plug and shell assembly may become disassembled or the relatively small and thin rigid shell may break or damage the syringe or the syringe needle during disassembly. In addition, various methods for mechanically mounting the pre-formed plug in the rigid plastic shell, for example, crimping, often result in particulates being formed due to the crimping process. In general, particulates are undesirable when assembling a needle shield, as the particulates may contaminate the relatively clean needle shield.
Alternative methods and assemblies for more securely mounting the pre-formed plug in the pre-formed shell complicate the construction of the needle shield. Needle shields that include the plug molded directly into the rigid shell result in an assembly with no air gap between the relatively soft plug and the hard, plastic shell. In situations where the needle of the syringe extends through the soft plug and impacts the hard, plastic shell, the needle tip may become damaged and/or contaminated. If needles that contact the hard, plastic shell are damaged and not detected prior to shipment to a user, the injection using the damaged needle tip may be quite painful for a patient.
During assembly of the needle shield with the syringe, each needle shield and syringe assembly is subjected to testing to determine if the needle tip has extended through the plug and has potentially impacted the hard, plastic shell or is otherwise damaged. The syringe and needle shield assemblies are commonly tested by passing each assembly through an electronic field which results in a spark from an electrical anode to the needle tip when the needle tip has extended through the soft plug. U.S. Pat. No. 6,229,314 B1 ('314 patent) discloses such an electronic field quality assurance test mechanism and method and is incorporated herein by reference. Needle shields that are co-injected into their shells are not adapted for such a test because no air gap is created between the plug and shell wherein a spark may be created at the needle tip.
However, the co-injected needle shields are extremely difficult to disassemble or break by removing the plug from the shell, which may sometimes happen in the mechanically assembled needle shields. Because the plug and shell are connected along an internal surface of the shell, no mechanical devices, clamps or fasteners are needed to secure the plug to the shell. Accordingly, the co-injected needle shield typically eliminates the risk of the plug becoming disassembled or falling out of the shell when the needle shield is removed from the shell. Therefore, the co-injected needle shield has the advantage of being rigidly secured to the shell and the disadvantage of being inappropriate for the electronic field quality assurance test. The mechanically assembled needle shield provides the advantage of being suitable for electronic field quality assurance testing because of the air gap between the plug and shell and the disadvantage of being dislodged from the shell during disassembly of the syringe from the needle shield or a complicated assembly process to more securely mount the plug to the shell.
In addition, the conventional co-injected needle shield restricts flexing of the plug, because the plug is rigidly held within the walls of the shell and is generally unable to flex radially outwardly due to being enclosed by the rigid shell. The restriction of the flexing of the plug in the co-injected needle shields typically increases the insertion and removal forces required to insert the needle tip into the plug and to remove the needle tip from the plug.
It would be desirable to construct a needle shield that includes an air gap between the soft plug and the hard shell, as is present in the mechanically assembled needle shield, such that the electrical field test is able to detect needles that extend completely through the plug while maintaining the advantage of rigid mounting of the plug to the shell in the co-injected needle shield to generally prevent the plug from becoming disengaged from the shell. It would also be desirable for the needle shield to have a relatively simple assembly that typically does not result in the formation of particulates.